How can I change the color of fly/walk mode crosshair?
This option is now supported, but not in time for version 2.70. Later versions will have this option and meanwhile you can download a daily build.

To change this setting go to: User Preferences > Themes > 3D View (on the left) and select View Overlay.

The option affects:
- the cursor (fly/walk and the default black part of the 3D cursor),
- the camera guides (enabled in Camera Panel > Display), and
- the stippled line guide that appears when scaling objects.